Re: Cabin temp
This is getting very strange. The problem is speed related, or maybe engine speed related. If the problem was a restriction in the heatercore lines, you would also have it at lower speeds. It looks almost if the coolant flow to the heater core gets less, or the blend doors are closing, but that should have no connection to the vehicle speed. Did you already select max temperature when driving on the highway? As soon as you slow down, the temperature gets higher again?
Just after the heatercore was replaced, the system was working properly?